Corrosion-inhibition of oxalic acid



May 21, 1957 M. A. SYTREICHER CORROSION-INHIBITION OF OXALIC ACID Filed April 15. 1954 2 Sheets-Sheet l NEQLT E 0: -6

INVENT OR Weight Loss in Grams Per Sq. 0m in 7 Hrs.

Michael A. Sfreicher ATTORNEY ay 1957 M. A. STREICHER 2,793,190

CORRCSION-INHIBITION OF OXALIC ACID Filed April 15. 1954 2 Sheets-Sheet Weight Loss in Grams Per Sq. Dm. in 7Hrs-. Michael Sfreicher BY W 7 ATTORNEY CORROSION-INHIBITION F OXALIC ACID Michael A. Streicher, Wilmington, D'eL, assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Application April 15, 1954, Serial No. 423,504

8 Claims. (Cl. 252-100) This invention relates to the corrosion inhibition of oxalic acid as regards stainless type steels and particularly to a method of inhibiting the corrosiveness in aqueous solution of oxalic acid by the incorporation of ferric ion.

The reactive properties of oxalic acid with respect to iron are well known, e. g., the removal of rust stains from fabrics by washing with oxalic acid in aqueous solution and other uses. More recently, oxalic acid has come into favor as a radiator cleaner for the purpose of dissolving out the hardness deposits precipitated from the cooling water and for the removal of the sludge, which contains a substantial amount of corrosion products derived from the interaction of the cooling water and the material from which the radiator is fabricated. Scant regard appears to have been given to the inhibition of corrosion resulting from the use of oxalic acid cleaners in automobile radiators, the objective being to obtain a thorough cleaning of the apparatus even though some incidental etching by the oxalic acid or other materials present therewith inevitably occurs. This may not be particularlydisadvantageous when oxalic acid is used for radiator cleaning purposes, for the reason that the cleanings are infrequent and some corrosion can be tolerated without deleterious results, but in other uses it is decidedly objectionable. Oxalic acid ha even more recently been employed to develop adherent coatings on stainless steels as a protection for the metal during deformation operations, such as drawing and the like, but this objective is entirely apart from the inhibition of corrosion on the part of the acid.

A great many uses of stainless steel exist wherein it is desirable to provide a cheap, quick, non-corrosive cleaner which may be employed'periodically to maintain the original bright surface of the metal for esthetic reasons. One use requiring considerable exterior cleaning exists in the case of stainless steel-sheathed railroad cars and diesel locomotives, although stainless steel display equipment in view of the purchasing public is another important situation where pleasing appearance is greatly sought after. 0n the other hand, there are numerous industrial processes, such as in the manufacture of textiles from polymeric materials, for example, wherein oxalic acid performs in the role of a process reactant and it i of the utmost importance that no carryover of discoloring or otherwise objectionable contaminants be tolerated, or that acid corrosion to the equipment with which it is' in contact be so excessive as to cause damage thereto.

A primary object of this invention is to provide a method of inhibiting the corrosiveness of oxalic acid in aqueous solution for stainless steels. Another object of this invention is to provide a method of inhibiting the corrosiveness of oxalic acid in aqueous cleaning solutions for stainless steels, while still retaining high cleaning'ability. Yet another object of this invention is to provide a method of inhibiting the corrosiveness of oxalic acid cleaning compositions for stainless steels which is cheap and not dangerous to using personnel. Another object of this invention isto provide a method of inhibiting the Patented May 21, 1957 corrosiveness of oxalic acid in aqueous solution, or of oxalic acid in aqueous solution containing other acids or acid salts hereinafter described, with respect to stainless steels which reduces or completely prevents objectionable contamination of the solutions during contact with stainless steel, and which also reduces or completely eliminates damage to stainless steel objects in contact with the solutions.

The manner in which the foregoing and other objects of this invention are obtained is disclosed in detail in the description hereinafter set forth and in the following drawings, wherein the graphing is in semilogarithmic representation with concentration of inhibitor plotted as the abscissa on the logarithmic scale, and in which:

Fig. 1 is a plot of the corrosion inhibition obtained according to this invention for AISI 300 series (18 Cr- 8 Ni) steels and substitute chrome-manganese steels in contact with 10% concentration boiling oxalic acid, and

Fig. 2 is a plot of the corrosion inhibition obtained according to this invention for representative AISI 400 series steels, including type 410, (13.5% Cr), type 43.0 (16% Cr) and type 446 (25% Cr) steels in contact with 10% concentration boiling oxalic acid.

Generally, the objects of this invention are obtained by incorporating in the aqueous oxalic acid cleaning composition or aqueous oxalic acid reactant a minor amount of ferric ion by addition to the acid of a ferric salt, preferably the oxalate, after which the oxalic acid is employed in its customary manner without regard to the fact that the inhibitor is present. The inhibited solution may also contain wetting and dispersing agents to facilitate cleaning, where this is the objective. In addition, the solution may contain sodium bisulfate either alone or in admixture with one or more of the group consisting of formic, acetic, sulfamic and glycolic acids, the inhibitory effect of ferric ion as regards sodium bisulfate being taught in my application S. N. 423,506 and as regards formic, acetic, sulfamic and glycolic acids in my application S. N. 423,505, both filed on the same date as the instant application.

Figs. 1 and 2 are graphic representations of the corrosion inhibition effect of ferric ion, added as ferric oxalate, with respect to different types of stainless steels when such steels are exposed to 10% concentration boiling oxalic acid aqueous solutions (boiling point about 101 C.). The 10% concentration was chosen for the tests hereinafter described because this concentration represents the limit of solubility of oxalic acid in water at room temperatures, i. e., 25 C. It is of interest that the corrosive action of oxalic acid decreases markedly with decreasing temperature, at C. falling off to only A of the rate for boiling 10% oxalic acid in the case of AISI type 304 steel, and decreasing even more with lower temperatures. This same phenomenon exists with respect to the AISI type 400 steels, except that the corrosion propensities of the latter are more pronounced than is'the case with the AISI type 300 steels, i. e., the 18-8 group.

The tests graphically represented in Figs. 1 and 2 were conducted over periods of 7 hours using 10% concentration boiling oxalic acid, which was maintained at this level of concentration by the use of a condenser which returned all material boiled off to the metal samplecontaining flask during the tests. Additional tests, vnot reported in detail herein, confirmed the corrosion inhibition of ferric ion for much greater periods of time, up to and beyond 60 hours. Referring to Fig. 1, it will be seen that theweight loss in g./sq. dm. during the 7 hour test periodfor type 304 steel decreased sharply in the presence of about 0.0035 g. Fe/l. of acid solution and then remained approximatepending on the particular composition.

ly constant until 0.036 was employed, when corrosion dropped very sharply to substantially zero for a concentration of 0.05 g./l. of acid solution. The surface of steel obtained with concentrations of iron from 0.05 to 10.0 was bright and complete inhibition was obtained throughout this entire range. In other'tests not reported in detail herein a lower limiting concentration. of 0.05 g. Fe/l. produced complete corrosion inhibition for AISI types 301, 302, 302B, 303, 304, 3041., 309, 316, 3'16L, 321, and 347 steels, the foregoing representations with respect to type 304 being equally applicable to these other steels as a group.

It has recently become necessary to use the so-called substitute stainless steels to conserve nickel for many uses which are considered non-essential to thenational defense, one. group of these comprising austenitic Cr-Mn steels,'i typified by the three compositions having the analyses 15% Crl7 Mn-l% -Ni, ;1-7.6% Cr-5.6% Mn-4.5% Ni, and 18% Cr-l5% Mn -0.5%- Ni, the balance being iron. The corrosion rate of the substitute stainless steels in uninhibited 'l0%'oxalic ac'id'rnay be 25 vtimes the rate of 18Cr 8Ni (AISI type-304)" steel.) As

shown in Fig. 1, the corrosion inhibition efiect of ferric ion I was ascertained with respect to these substitute stainless steels and it was found thatcomplete inhibition was obtained with each, the lower limitingconcentration of ferric ion ranging fromabout-0.0 6 g./l. to 0.12 gL/l. de-

It Was found that, onthe basis of corrosion tests for numerous heats of substitute stainless steels of the general composition -detailed hereinabove, that the lower limiting concentration of ferric ion necessary to obtain substantially complete inhibition varied somewhat for different samples. However,.in.all cases total inhibition was attained at concentrations offerric ion above 0.25 g./l. and, therefore, this limit is preferred for assured protection. In all instances the surfaces of the exposed steels were maintained in their original brightness, no visible coatings being formed.

Referring to Fig. 2, the corrosion inhibition of oxalic acid by the ferric ion is delineated for three individual members of the 400 AISI series of stainless steels, these being iron alloyed with chromium solely, without any nickel. As shown in Fig. 2, the inhibition effect of ferric ion for AISI type 410 (0.15% C max., 11.513.5% Cr, balance iron) is similar to that for AISI type.304, except that substantially complete corrosion inhibition is not obtained until approximately 2 grams, and preferably'3, of iron per liter of concentration boiling oxalic acid is used. With this alloy'it was found that a heavy, adherent greenish coating developed on the metal surface in the course of exposure to the hot acid. In contrast, the corrosion inhibition of AISI type 430 steel (0.12% C max., l4.018.0% Cr, balance iron), depicted in Fig. 2, is substantially zero until approximately 0.36 g. Fe/L of acid solution is utilized, when corrosion falls off very quickly upon increase of the concentration of iron to about 0.5 g./l., when substantially zero corrosion is attained for higher concentrations of iron. With this steel a smooth adherent light black coating is sometimes developed gradually on the metal after contact with the acid of a duration of 2-3 hours, which coating appears to have a decorative appeal but which would, of course, be objectionable if it were desired to maintain the metal in its original shiny state. i i

As shown in Fig. 2, the corrosion of AISI type 446 steel (0.35% C max., 0.25% Na max, 1.5% Mn max., 23.0-27.0% Cr, balance iron) is seen actually to increase from about 0,1 g./ sq. dm. in 7 hours exposure with addition of ferric ion up to a level of addition of about 0.0025 g./l., thereafter decreasing to a point lower than that for substantially uninhibited acid. at 0.01, complete inhibition being finally attainedat 0.05. g./l'., and .at.concentrations excess of this level. The completely inhibited acid preserved a bright surface in the region above 0.05.

As hereinabove mentioned, it is preferred to add the ferric ion to the oxalic acid as the oxalate, although corrosion inhibition can also be achieved if ferric ion is incorporated through another salt, such as the nitrate, sulphate, chloride and bromide, or mixtures thereof, for example. The anions for the halide salts appear to have a corroding action on the stainless steels entirely'independent of the corrosion inhibiting; action ofthe ferric ions and, therefore, my tests have'disclosed that, above a certain limit of concentration of these salts, an acceleration of corrosion actually results, which increases progressively with concentration of the saltup to and above the corrosion existing with oxalic acid containing no additives. I have found that this corrosion inhibitioncounteracting effect of the halide anions is not disadvantageous, for the reason thatthe limiting, concentration of ferric ion necessary for completeinhibitionof-coP rosion is only of the order of 25 %33,%j of that at which net corrosion acceleration due ,tojthe anions occur,- so that all of the benefits of this invention are still obtainable, provided that the proper concentration of additive is employed. In summary, if'R ;mols.oxalic acid/mols ferric chloride, for example, I have. found that the minimum amount of ferricchloride'required'forcomplete inhibition of 10% concentration boiling oxalic acidi corresponds to R=820, while for 30% concentration boiling oxalic acid the minimum amount of ferric chloride required for complete inhibition corresponds to R=2450. In contradistinction, the. amount of ferric chloride which canbe tolerated while still preserv- "ing maximum inhibition in 10% concentration boiling oxalic acid, corresponds to R=270 (or greater), and a similar relationship exists for more concentrated acid solutions. As a practical matter, contact of the stainless steels with chloride salts is objectionablefor the reason that stress corrosion results therefrom as a phenomenon completely independent of the acid corrosion'with which this invention is concerned. Accordingly, the use of chloride salts even in amounts proportioned as hereinbefore described is not recommended. V V The mechanism responsible for the corrosion inhibi "tion of oxalic acid towards stainless steels by the ferric ion, while keeping the surface bright, is not understood; however, the efiect appears to result from a specific prop 'erty of the ion itself, since tests with nickel, chromium, manganese, and aluminum salts showed that no detectable inhibition of corrosion occurred with any of these materials. In the case of cupric, ceric and stannic ions, there'was a reduction of corrosiongbut not complete inhibition or maintenance of the original bright finish. Copper acts quite diiferentlythan the ferric ion, in that it plates out and redissolves. Tests with aluminum chloride revealed that the aluminum ion displays no corrosion inhibitory elfect for oxalic acid in contact with either the 300 or 400 AISI series steels, a mol ratio of 500 mols of oxalic acid to mols of aluminum chloride increasing the corrosion of both series of steels by a'factor of about 34% over oxalic acid containing no additives whatsoever, while mol ratios of 9.5 and increased .corrosion by'factors of 3 and 10, respectively, on the same basis as that applicable tothe ferric ion tests already reported. V v

While the data reported hereinabove islargelyconcerned with tests conducted with 10 concentration boiling oxalic acid, I have found that ferric ion inhibits corrosion toward more concentrated oxalic acid, such as. aqueous solutions in the range of 60%-70% concentration, the same limits of ferric ionjhereinabovereported affordingcomplete inhibition regardless of varia' tions in acid concentration. .Furthermore, my' tests have shown that ferricion inhibition is effective against sensitized steels as. well as those whichhav'e not been sension the samples tized, corrosion inhibition testson 304, 304L, 309, and 316 steels after heat treatments of 1 hour duration at 1,250" P. showing no deviation from the pattern reported for nonsensitized steels. The latter steels precipitate chromium carbides at grain boundaries during sensitization; however, sensitization of a 316L steel, which produces no carbides or other detectable phase at grain boundaries, but which possesses a high intergranular nitric acid corrosion rate, causes it to behave in the same manner as the sensitized steels as regards intergranular attack. Corrosion-inhibited oxalic acid tests of stainless steels showing this normally characteristic type of attack revealed complete inhibition under microscopic examinations of 500 X.

From the foregoing it will be understood that my invention comprises an effective method of inhibiting corrosion for oxalic acid with respect to the stainless steels, within which term it is intended to comprehend both the conventional stainless steels and the so-called substitute stainless steels, and that the limiting concentrations which are employed depend upon the specific analysis of the stainless steel involved and the salt through which the ferric ion is introduced, both of which are subject to relatively Wide modification, for which reasons it is intended to be limited only by the following claims.

What is claimed is:

l. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about 70% oxalic acid toward stainless steels comprising incorporating in said solution a minor quantity of a ferric ion-contributing salt other than an iron halide, but not less than an amount yielding about 0.05 gram/liter of ferric iron, and thereafter contacting said stainless steels with said solution.

2. The method according to claim 1 in which said ferric-ion contributing salt consists of ferric oxalate.

3. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about 70% oxalic acid toward AISI series 300 stainless steels comprising incorporating in said solution a quantity of a ferric ion-contributing salt other than an iron halide furnishing not less than about 0.05 gram of iron/liter of said solution and thereafter contacting said stainless steels with said solution.

4. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about 70% oxalic acid toward AISI series 400 stainless steels comprising incorporating in said solution a quantity of a ferric ion-contributing salt other than an iron halide furnishing a minimum of about 0.01 to 3.0 gram of iron/liter of said solution depending upon the composition of the particular steel which is contacted with said solution and thereafter contacting said stainless steels with said solution.

5. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about oxalic acid toward Cr-Mn-Ni stainless steels comprising incorporating in said solution a quantity of a ferric salt other than an iron halide furnishing a minimum of 0.06 to 0.25 gram of iron/liter of said solution depending upon the composition of the particular steel which is contacted with said solution and thereafter contacting said steels with said solution.

6. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about 70% oxalic acid toward a stainless steel having a composition in which the carbon content is a maximum of about 0.12%, the chromium content is in the range of about 14.0 to 18.0% and the balance is iron comprising incorporating in said solution a quantity of a ferric salt other than an iron halide furnishing in excess of about 0.5 gram of iron/liter and thereafter contacting said stainless steel with said solution.

7. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up to and including about 70% oxalic acid toward a stainless steel having a composition in which the carbon content is a maximum of about 0.15%, the chromium content is in the range of about 11.5 to 13.5% and the balance is iron comprising incorporating in said solution a quantity of a ferric salt other than an iron halide furnishing in excess of about 3.0 gram of iron/liter of said solution and thereafter contacting said stainless steel with said solution.

8. The method of substantially inhibiting the corrosion of oxalic acid in aqueous solution in concentrations up 'to and including about 70% oxalic acid toward a stain less steel having a composition in which the carbon content is a maximum of about 0.35%, the nitrogen content is a maximum of about 0.25%, the manganese content is a maximum of about 1.5%, the chromium content is in the range of about 23.0 to 27.0%, and the balance is iron comprising incorporating in said solution a quantity of a ferric salt other than an iron halide furnishing in excess of about 0.01 gram of iron/liter of said solution and thereafter contacting said stainless steel with said solution.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,950 Roscnfeld et a1. Mar. 17, 1953 

1. THE METHOD OF SUBSTANTIALLY INHIBITING THE CORROSSION OF OXALIC ACID IN AQUEOUS SOLUTION IN CONCENTRATIONS UP TO AND INCLUDING ABOUT 70% OXALIC ACID TOWARD STAINLESS STEELS COMPRISING INCORPORATING IN SAID SOLUTION A MINOR QUANTITY OF A FERRIC ION-CONTRIBUTING SALT OTHER THAN AN IRON HALIDE, BUT NOT LESS THAN AN AMOUNT YIELDING ABOUT 0.05 GRAM/LITER OF FERRIC IRON, AND THEREAFTER CONTACTING SAID STAINLESS STEELS WITH SAID SOLUTION. 